Method for producing (meth)acrylic esters

ABSTRACT

Basic (meth)acrylates IV are prepared by transesterification of alkyl (meth)acrylates I in the presence of a catalyst and working-up of the reaction mixture by distillation, by a process in which a gas or gas mixture which is inert under the reaction conditions is passed through the reaction zone and/or heat exchanger.

[0001] The present invention describes a process for the preparation ofbasic (meth)acrylates (IV) in high purity and with a high yield bytransesterification of an industrial lower alkyl (meth)acrylate I withbasic alcohols (R²OH).

[0002] High purity is understood as meaning a purity of at least 99.8%,the total content of saturated impurities (without water) being not morethan 1000 ppm and the content of N,N′-dimethylpiperazine, ethyleneglycol di(meth)acrylate and vinyloxyethyl (meth)acrylate being not morethan 100 ppm in each case.

[0003] (Meth)acrylates are useful starting compounds for the preparationof polymers and copolymers which are used, for example, as finishes,dispersions or adhesives.

[0004] Saturated impurities, i.e. those which have no carbon-carbonmultiple bonds, e.g. alcohols, ethers and acetic and propionic acidderivatives, are disadvantageous in that they withstand thepolymerization unchanged, i.e. are not incorporated into the polymer,and may lead to the product having an annoying odor. In order toseparate off the residual volatile components, for example fromdispersions, expensive treatments (deodorizations) are additionallyrequired.

[0005] As a rule, a treatment referred to as physical deodorization iscarried out and comprises stripping the dispersion with steam, air ornitrogen in a stirred container (German Published Application DAS 12 48943) or in a countercurrent column. The treatment is carried out in oneor more stages, depending on the amount and the boiling points of thecomponents to be separated off. The removal of these impurities isaccordingly an expensive procedure, which moreover cannot be carried outin the case of heat-sensitive dispersions owing to the thermal stress.

[0006] The content of ether (dibutyl ether is entrained, for example,with butyl acrylate as lower (meth)acrylate I, see below) also has anadverse effect if the procedure is carried out in the presence ofoxygen-containing gases, such as air, for example for stabilization. Itis known that, in the presence of oxygen, ethers very readily formperoxides and it is known that these may then initiate a polymerizationof (meth)acrylate compounds, which may even take place explosively.

[0007] The ether content is accordingly not only a quality problem butalso a safety problem.

[0008] Ethylene glycol di(meth)acrylate and vinyloxyethyl(meth)acrylate, which occur as secondary components in the preparationof alkylaminoethyl (meth)acrylates, contain two unsaturated groups andtherefore act as crosslinking agents in the polymerization. This isextremely disadvantageous since consequently the polymerization and thequality of the polymers are adversely affected, for example by gelformation. In addition, they influence the shelf-life.

[0009] The preparation of basic (meth)acrylates IV bytransesterification of lower (meth)acrylates I with basic alcohols R²OHis generally known.

[0010] It is furthermore generally known that the transesterification isan equilibrium reaction. In order to achieve economical conversions, theresulting lower alkanol R¹OH, being the component with the lowestboiling point, is therefore generally removed continuously from theequilibrium by distillation, a very pure alkanol fraction beingdesirable for economic reasons in order to be able to use it again, forexample in the preparation of the lower (meth)acrylate I byesterification. Owing to the position of the boiling points andformation of azeotropic mixtures, however, this distillate generallydoes not consist of pure lower alkanol R¹OH but is contaminated with thelower (meth)acrylate I and possibly with basic alcohol R²OH.

[0011] Since, for economic reasons, it is expedient to utilize thedistillate, impurities have an adverse effect, particularly when theyare basic impurities, i.e. compounds having an amino group.

[0012] The particularly economical recycling to the synthesis of thelower ester is especially influenced thereby, cf. for example EP-A2 906902, page 3, lines 4-16.

[0013] EP-A2 906 902 describes a process for the preparation ofalkylamino (meth)acrylates by transesterification of alkyl(meth)acrylates with alkylaminoalcohols in the presence of a catalyst,e.g. dibutyltin oxide, in which the alcohol-containing distillate(azeotropic mixture) is passed, either directly or after a furtherdistillation, over an acidic ion exchange resin. The basicnitrogen-containing impurities from the distillate are bound by theacidic groups and thus separated from the alkanol/(meth)acrylatemixture, which can then be used again in the synthesis of the lower(meth)acrylate. The working-up of the transesterification mixture iscarried out in a plurality of distillation stages, the additionalformation of the Michael adducts during the catalyst removal beingreduced as far as possible.

[0014] Michael adducts are defined as the compounds formed by additionof alcohols at the double bond of the (meth)acrylates.

[0015] It is generally known that this addition (cf. equation I) takesplace in particular in the presence of alkaline catalysts (Organikum,17th Edition, page 506, VEB Deutscher Verlag der Wissenschaften, Berlin1988).

[0016] By means of a two-stage catalyst removal (EP-A2 906 902, page 4,lines 51-57), the additional formation (ratio of increase) of theMichael adducts is kept below 2%. According to examples III-1, III-2 andIII-3, the distillation temperatures and the residence times play adecisive role. On increasing the temperature in stage 2 or the residencetime in both stages, the ratio of increase of the Michael adducts doesin fact increase substantially (from 0.48 to 0.96 and 2.2%,respectively).

[0017] The actual, absolute content of the Michael adducts in thereaction mixture, which is decisive for the yield and thecost-efficiency of the process, is mentioned nowhere in EP-A 906 902.

[0018] The process has the following disadvantages:

[0019] 1. Necessity of purification using an ion exchanger.

[0020] 2. The regeneration and disposal of the exchange resin laden withthe basic impurities is expensive and environmentally polluting.

[0021] 3. It requires from 5 to 7 distillation steps and is thustechnically complicated.

[0022] 4. The yield is low (about 33%, example III-1).

[0023] 5. The amino alcohol must be metered in continuously over a longperiod (4 hours, cf. example III-1) in order to reduce the formation ofthe Michael products.

[0024] 6. Long reaction times are required (7-8 hours) which reduces thecost-efficiency.

[0025] Our own investigations have shown that in particular the reactiontime (i.e. the residence time in the reactors) has a decisive effect onthe formation of the Michael adducts (cf. example 3). On the other hand,the temperature and the residence time in the catalyst removalsurprisingly are of no importance for the formation of the Michaeladducts in the novel process (cf. comparative examples 1 and 2).

[0026] As a rule, methyl and ethyl (meth)acrylate are used as startingmaterials in the transesterification (EP-A 960 877, FR 2 617 840),whereas butyl (meth)acrylate is regarded as being disadvantageous owingto its high boiling point (U.S. Pat. No. 2,832,800, column 2, lines60-70).

[0027] Since the literature on the transesterification of alkyl(meth)acrylates provides no detailed information about the accompanyingsubstances and impurities in the starting esters used, it must beassumed that the purity of the (meth)acrylates used is very high and notroublesome components are present.

[0028] However, the use of esters of high purity is disadvantageoussince they have to be purified in a technically complicated manner bydistillation after their preparation. In view of the generally knownhigh tendency of (meth)acrylate compounds to polymerize under thermalstress, this is particularly disadvantageous.

[0029] In particular, titanium alcoholates whose alkyl groups are C₁- toC₄-alkyl radicals, e.g. tetramethyl, tetraethyl, tetraisopropyl,tetrapropyl, tetraisobutyl and tetrabutyl titanates, are proposed ascatalysts for the preparation of (meth)acrylates by transesterification(cf. for example EP-B1 298 867, EP-A2 960 877). Inter alia, titaniumphenolates (German Laid-Open Application DOS 200 86 18), metal chelatecompounds of, for example, hafnium, titanium, zirconium or calcium,alkali metal and magnesium alcoholates, organic tin compounds or calciumand lithium compounds, for example oxides, hydroxides, carbonates orhalides thereof, are furthermore proposed as catalysts.

[0030] For economic and ecological reasons, in particular alkyltitanates are used, although they are, for example, sensitive to eventraces of water and some titanium alcoholates are unstable at relativelyhigh temperatures. The result is fouling (see below) of the apparatuswalls.

[0031] Furthermore, it is generally known that alkyl titanates promotethe polymerization of (meth)acrylates and may therefore give rise to theformation of polymer during the transesterification and the working-upof the transesterification mixture (German Laid-Open Application DOS 2008 618, page 3, German patent 1,067,806, column 1, lines 39-41).

[0032] Another disadvantage is that, because their activity isrelatively low in some cases, titanium alcoholates necessitate hightransesterification temperatures in order to achieve economicalconversions or reaction times (EP-A 160 427, page 2, lines 23-32). Thisin turn can lead to increased polymer formation and fouling.

[0033] Another problem is the loss of activity suffered by titaniumalcoholates in the course of time (German Laid-Open Application DOS 2805 702, page 5, lines 12-21). In order to achieve economicalconversions, the amount of catalyst must be increased and/or thereaction time lengthened. In view of the instability of the titanatesand the byproduct and polymer formation, this is known to bedisadvantageous.

[0034] Added to this is the fact that (meth)acrylic acid compounds havea considerable tendency to polymerization, very particularly if heatacts on them. Especially on the preparation and the distillativepurification, they are exposed to temperatures which can easily initiatean undesired polymerization. The use of polymerization inhibitors, as isgenerally recommended, also cannot completely prevent the polymerformation.

[0035] Soiling of the apparatuses, blockage of pipes and pumps andfouling of column trays and heat exchanger surfaces are as a rule theresult of polymer formation. The cleaning of the plants is acomplicated, expensive and environmentally polluting process, and theyield and the availability of the plants (run time) are also greatlyreduced as a result.

[0036] JP-A 3-112 949 describes a process for the preparation ofdimethylaminoethyl acrylate by transesterification of n-butyl acrylatewith dimethylaminoethanol using tetra-n-butyl titanate, in which thepurification of the reaction mixture is effected in the absence ofoxygen.

[0037] However, the yields of less than 90% are a disadvantage of thisprocess.

[0038] It has now been found that the abovementioned problems in aprocess for the preparation of basic (meth)acrylates IV bytransesterification of alkyl (meth)acrylates I in the presence of acatalyst and working-up of the reaction mixture by distillation arereduced if a gas or gas mixture which is inert under the reactionconditions is passed through the reaction zone and/or heat exchanger.

[0039] The reaction can be represented by equation II:

[0040] Here,

[0041] R is hydrogen or methyl,

[0042] R² is a straight-chain or branched, saturated or unsaturatedalkyl radical comprising two to twelve carbon atoms and substituted byat least one NR³ ₂ group and

[0043] R³ is a straight-chain or branched, saturated or unsaturatedalkyl radical comprising two to six carbon atoms, it being possible forN with the substituents R³ also to form a five- to seven-membered ringand for the substituents R³ to be identical or different.

[0044] R¹ should contain at least one carbon atom less than R².

[0045] Lower alkyl (meth)acrylate I is, for example, a (meth)acrylate ofan alcohol of one to six carbon atoms, e.g. of methanol, ethanol,isopropanol, n-propanol, n-butanol, isobutanol, sec-butanol,tert-butanol, n-pentanol or n-hexanol, preferably of methanol, ethanolor n-butanol, particularly preferably of n-butanol. n-Butyl acrylate isparticularly preferred.

[0046] Preferred basic alcohols R²OH are 2-dimethylaminoethan-1-ol,3-dimethylaminopropan-1-ol, 1-dimethylaminopropan-2-ol,2-dimethylaminopropan-1-ol, 6-dimethylaminohexan-1-ol,2-diethylaminoethan-1-ol, 3-diethylaminopropan-1-ol,6-diethylaminohexan-1-ol, 2-dibutylaminoethan-1-ol,3-dibutylaminopropan-1-ol and 6-dibutylaminohexan-1-ol, saiddialkylaminoethanols being particularly preferred, especially2-dimethylaminoethan-1-ol.

[0047] Here, high purity is understood as meaning a purity of at least99.8%, the total content of saturated impurities (without water) beingnot more than 1000 ppm, preferably up to 500 ppm, and that ofN,N′-dimethylpiperazine, ethylene glycol di(meth)acrylate andvinyloxyethyl (meth)acrylate being not more than 100, preferably notmore than 50, particularly preferably not more than 20, in particularnot more than 10, ppm in each case.

[0048] Industrial alkyl acrylate and industrial alkyl methacrylate areunderstood as meaning the lower (meth)acrylates I produced on anindustrial scale and having, as a rule, a purity of 99.0-99.8%. These(meth)acrylates contain as impurities substantially dialkyl ether(0.01-0.2%), alkyl acetate (0.01-0.1%), alkyl propionate (0.02-0.1%),alkanol R¹OH (0.01-0.05%), water (0.01-0.05%), (meth)acrylic acid(0.001-0.1%) and others, e.g. isomeric alkyl acrylates (0.01-0.3%).

[0049] The novel process is described below by way of example forn-butyl acrylate as I without being restricted to it and can be appliedin an analogous manner also to lower alcohols R¹OH other than n-butanol,for example C₁- to C₆-alcohols, e.g. methanol, ethanol, isopropanol,n-propanol, isobutanol or n-pentanol. For this purpose, the relevantalcohol or the relevant lower (meth)acrylate I is used instead ofn-butanol, the operating parameters being adapted to the changed boilingpoints and other physical properties of the starting materials andproducts, which is possible on the basis of experiments conventional inthe field.

[0050] The novel process can in principle also be applied in ananalogous manner to methacrylates.

[0051] The lower (meth)acrylate I is used in the novel process inindustrial purity, as stated at the outset.

[0052] The lower (meth)acrylate I used in the transesterificationgenerally has, in industrial purity, a composition as described above.

[0053] Of course, a lower (meth)acrylate I having a higher purity, forexample up to 99.95% by weight, can also be used, in which case theother impurities are present in a correspondingly smaller amount.

[0054] The higher alcohol R²OH usually has a purity of at least 99.0% byweight and a water content of 0.01-0.2% by weight.

[0055] The ethylene glycol content in the case of dialkylaminoethanolsas higher alcohol R²OH should be not more than 100 ppm, preferably notmore than 50 ppm, particularly preferably not more than 20 ppm, inparticular not more than 10 ppm. The vinyloxyethanol content in thehigher alcohol R²OH should be not more than 100 ppm, preferably not morethan 50 ppm, particularly preferably not more than 20 ppm, in particularnot more than 10 ppm.

[0056] Dialkylaminoethanols may also contain traces, for example up to200 ppm, preferably less than 100 ppm, of higher homologs.

[0057] The N,N′-dimethylpiperazine content is as a rule not more than100 ppm, preferably not more than 50 ppm, particularly preferably notmore than 20 ppm, in particular not more than 10 ppm.

[0058] Mixtures of higher alcohols may also be used for thetransesterification.

[0059] The transesterification can be carried out in a manner known perse, for example as follows:

[0060] The lower (meth)acrylate I, preferably the methyl, ethyl orn-butyl ester, particularly preferably the n-butyl ester, is reactedwith the higher alcohol R²OH in a molar ester:alcohol ratio of 1:1-4:1in the presence of at least one catalyst.

[0061] The transesterification can be carried out batchwise,semicontinuously or continuously, preferably continuously.

[0062] For the novel process, it is possible to use alltransesterification catalysts described in the prior art, preferablytitanium, magnesium or aluminum alcoholates, particularly preferablytitanium alcoholates, in particular titanium alcoholates of the alcoholspresent in the transesterification, R¹OH and R²OH.

[0063] If the titanium catalyst used is a lower alcoholate which doesnot contain the alcohol component R¹OH, in a preferred embodiment thecatalyst is prepared beforehand as follows:

[0064] A lower titanium alcoholate Ti(OR⁴)₄, preferably theisopropylate, isobutylate or n-butylate, is reacted with the higheralcohol R²OH (cf. equation III) at elevated temperatures (50-130° C.).The higher alcohol R²OH is used in a molar excess (as a rule from 1:5 to1:20).

[0065] R² cf. equation II

[0066] R⁴ is C₁-C₈-alkyl, preferably isopropyl, isobutyl or n-butyl

[0067] R²OH and R⁴OH should preferably fulfill the following conditionwith regard to their boiling points bp.:

bp.(R²OH)≧bp.(R⁴OH)+20° C.

[0068] Under these conditions, it is technically simple to keep thelosses of R²OH small and to separate off R⁴OH as completely as possible.

[0069] The alcohol R⁴OH formed in the reaction is separated off bydistillation or rectification, if necessary under reduced pressure. Thiscan, if required, be supported by stripping with a suitable, unreactivegas. The resulting residue is the catalyst solution for thetransesterification (Ti content: 2-10% by weight) and contains, as arule, less than 400 ppm of R⁴OH. Consequently, virtually no foreignalcohol (R⁴OH) is introduced into the transesterification mixture (<100ppm in the mixture).

[0070] However, it is of course also possible for the catalyst solutionto contain mixed titanium alcoholates, depending on the reactionaccording to equation III.

[0071] With the use of a titanate, for example one prepared by theabovementioned method, the titanium content in the reaction mixture isas a rule 0.01-1% by weight.

[0072] The transesterification is carried out in one reactor or in aplurality of reactors connected in series, having at least one attachedrectification column and condensers.

[0073] The reaction temperature is as a rule 80-140° C., preferably from100 to 130° C., and the pressure is from 200 mbar to atmosphericpressure, preferably 300-800 mbar, particularly preferably from 400 to600 mbar.

[0074] In the case of a plurality of reactors, the temperature in thevarious reactors may be identical or different, for example theirincrease or decrease, preferably increase, from one reactor to theother.

[0075] Heat can be supplied via wall heating and/or external or internalheat exchangers, for example tubular or plate heat exchangers,preferably via external circulation evaporators. The rectificationcolumns are of known design and have internals having separationactivity (for example bubble trays, Thormann trays, valve trays, sievetrays or dual-flow trays) or contain dumped or stacked packings. Thecondensers are likewise of known design and can be operated indirectly,for example as tubular or plate heat exchangers, or directly, forexample as quench coolers. The uniform thorough mixing of the reactionsolution is effected in a known manner, for example by stirring, pumpedcirculation or forced or natural circulation, preferably by forced ornatural circulation.

[0076] The reaction zone and/or the heat exchangers installed in theplant, for example of the distillation units or reactors, is/arecontinuously flushed, according to the invention, with a gas or gasmixture which is inert under the reaction conditions, for examplenitrogen, air, nitrogen/oxygen mixtures, argon, helium, carbon dioxideor carbon monoxide, preferably air or air/nitrogen mixtures, inparticular those having an oxygen content of from 0.1 to 15, preferablyfrom 0.5 to 10, % by volume and very particularly preferably thoseair/nitrogen mixtures having an oxygen content of from 1 to 5% byvolume. The purge gas is preferably passed through the reaction mixtureor along the heat exchanger surfaces present, particularly preferably ina forced or natural circulation evaporator present.

[0077] For this purpose, the purge gas is metered in, with pressure orvolume regulation via suitable feed apparatus known per se and notrestricted, in the vicinity of the heat exchanger surface present, sothat the, preferably continuous, purge gas stream is fedcountercurrently or cocurrently relative to the liquid along the heatexchanger surface.

[0078] The purge gas can be preheated to the temperature of the heatexchanger medium so that the temperature of the purge gas differs, forexample, by not more than 15° C., preferably not more than 10° C., fromthe temperature of the heating medium.

[0079] In each case 0.1-100, preferably 0.2-80, particularly preferably0.5-70, in particular 1-50, parts by volume, based on the volume of thereaction mixture (=1 part by volume) in the reactors and postreactors inthe reaction zone, of purge gas are passed per hour through the heatexchangers or the reaction zone.

[0080] Particularly preferably, the purge gas is passed via the heatexchangers in which the reaction medium in the reactors or in thedistillation columns is heated.

[0081] A particular embodiment of the novel transesterificationcomprises carrying out the reaction in at least one reactor withattached column and passing the reaction mixture continuously into apostreactor which is connected on the gas side to a transesterificationreactor, preferably the last transesterification reactor, or to theattached column.

[0082] The temperature in the postreactor is as a rule 1-10° C. higherthan in the reactor.

[0083] The residence time in the reaction zone, comprising thereactor(s) and, if required, the postreactor(s), is 1-4, preferably1.5-3, hours.

[0084] The column(s) attached to the reactor(s) has/have as a rule 10-30theoretical plates. The reflux ratio is as a rule 5-20:1, preferably7-15:1. The conditions of this distillation are generally chosen so thatthe butanol fraction at the top of the column attached to the reactorcontains 5-30%, preferably 10-20%, of n-butyl (meth)acrylate. As a rule,not more than 1, preferably not more than 0.5, particularly preferablynot more than 0.3, % by weight of the higher alcohol R²OH is present.

[0085] The lower alcohol R¹OH liberated during the transesterificationis separated off together with a part of the lower (meth)acrylate I viathe top of the rectification columns attached to the reactors.

[0086] The distillation conditions, for example the theoretical platesand the reflux ratio, are preferably chosen so that a nonazeotropicmixture is taken off at the top of the column, in which mixture thecontent of lower (meth)acrylate I is higher compared with the azeotropiccomposition comprising lower alkanol R¹OH and lower (meth)acrylate Iunder the corresponding conditions.

[0087] The distillate can be recycled directly, i.e. without anadditional purification step, into the synthesis of n-butyl(meth)acrylate, where it can be reacted with (meth)acrylic acid again togive the starting ester I, as described in the German Patent Applicationwith the title Preparation of (meth)acrylates, with the same date offiling as the present document and the Application number 101 27 941.8.Advantageously, it can be fed there to the working-up process,particularly preferably to an extraction process.

[0088] The columns can be stabilized using the conventional stabilizersor mixtures thereof, for example N-oxyls, such as4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl or4-oxo-2,2,6,6-tetramethylpiperidin-N-oxyl, phenols and naphthols, suchas p-aminophenol, p-nitrosophenol, 2-tert-butylphenol,4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-methyl-2,6-tert-butylphenol(2,6-tert-butyl-p-cresol) or 4-tert-butyl-2,6-dimethylphenol, quinones,such as hydroquinone or hydroquinone monomethyl ether, aromatic amines,such as N,N-diphenylamine, phenylenediamines, such asN,N′-dialkyl-para-phenylenediamine, it being possible for the alkylradicals to be identical or different and in each case, independently ofone another, to be of 1 to 4 carbon atoms and to be straight-chain orbranched, hydroxylamines, such as N,N-diethylhydroxylamine,phosphorus-containing compounds, such as triphenylphosphine, triphenylphosphite or triethyl phosphite, or sulfur-containing compounds, such asdiphenyl sulfide or phenothiazine.

[0089] Furthermore, degradation products or derivatives of stabilizersmay also be used, for example the Michael adduct of (meth)acrylic acidor (meth)acrylates and hydroquinone.

[0090] The stabilization can be effected in the presence or absence ofmolecular oxygen, preferably in the presence thereof.

[0091] The stabilization is preferably effected using phenothiazine,hydroquinone, hydroquinone monomethyl ether,4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl,4-oxo-2,2,6,6-tetramethylpiperidin-N-oxyl, 2,6-tert-butyl-p-cresol ormixtures thereof in amounts of, in each case, from 10 to 5000 ppm,particularly preferably phenothiazine or a phenothiazine-containingmixture, in particular aphenothiazine/4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl mixture,being used for the stabilization. The addition can be effected in eachcase by the starting materials, directly or via the recycle or refluxstreams.

[0092] In particular, the stabilization is effected using the reflux towhich 100-1000 ppm of phenothiazine and 10-500 ppm of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl have been added.

[0093] The stabilization is preferably effected using a solution of thisstabilizer mixture in the lower (meth)acrylate.

[0094] Particularly preferably, the dissolved stabilizer mixture issprayed onto the condenser surfaces.

[0095] The reaction mixture formed is discharged from the reactor,preferably continuously, and separated in a distillation unit (catalystremoval) into a top product, which mainly contains the desired ester andthe starting materials, and a bottom product, which substantiallycontains desired ester, catalyst, high-boiling byproducts andpolymerization inhibitors. The bottom product can be recycled at leastpartly, preferably in an amount of from 50 to 100%, into the firstreactor. Any residue present can be subjected, for example, to a residuetreatment.

[0096] The distillation unit may consist, for example, of a conventionalforced or natural circulation evaporator and a column, for examplehaving 5-15 theoretical plates, of conventional design. Therectification columns of the novel process are of known design and haveinternals having separation activity (for example bubble trays, Thormanntrays, valve trays, sieve trays or dual-flow trays) or contain dumped orstacked packings.

[0097] According to the invention, the evaporator surfaces of thecatalyst removal stage may also be flushed with a purge gas as describedabove.

[0098] The bottom temperature is, as a rule, 80-160° C., preferably90-150° C., particularly preferably 90-120° C., and the correspondingpressure is 20-500, preferably 50-300, particularly preferably 80-150,mbar. The reflux ratio is as a rule 5:1-1:5, preferably 3:1-1:3,particularly preferably from 2:1 to 1:2.

[0099] The distillation can also be carried out at a higher temperaturein order to cleave Michael adducts, as described in the German PatentApplication with the title Preparation of (meth)acrylates with the samedate of filing as the present document and the Application number 101 27939.6.

[0100] If required, the distillation process can be supported by passingthrough a gas stream as described above which is substantially inertunder the reaction conditions (stripping), e.g. nitrogen, as well as anoxygen-containing gas, such as air or air/nitrogen mixtures, inparticular those having an oxygen content of from 0.1 to 15, preferablyfrom 0.5 to 10, % by volume and very particularly preferably thoseair/nitrogen mixtures which have an oxygen content of from 1 to 5% byvolume. The passage of the purge gas according to the invention ispreferably associated with the stripping process.

[0101] In order to prevent polymer formation in the distillation unit,advantageously an about 0.1 to 1% strength solution of phenothiazine inthe starting ester is sprayed into the condensers. The stabilization ispreferably effected using the reflux to which 100-1000 ppm ofphenothiazine and 10-500 ppm of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl have been added.

[0102] Particularly preferably, the dissolved stabilizer mixture issprayed onto the condenser surfaces.

[0103] The distillate containing the desired ester and obtained in thecatalyst removal can be separated in a further distillation unit(purifying distillation) into a low boiler fraction, which mainlycomprises the starting materials and can be recycled into thetransesterification, into a high boiler fraction (bottom product), whichmainly contains desired ester and inhibitors and is advantageouslyrecycled into the first distillation unit but can also be partly fed tothe residue treatment, and into a medium boiler fraction, whichsubstantially contains the desired ester. The desired ester isdischarged, preferably in gaseous form, via a side take-off in the lowercolumn region, preferably in the lower half, particularly preferably inthe lower third, and is condensed. The desired ester is stabilized with10-20 ppm of hydroquinone monomethyl ether or 2,6-tert-butyl-p-cresol,preferably by spraying onto the condenser in the gaseous side take-off.

[0104] The column has, as a rule, 10-30 theoretical plates.

[0105] The condensers and evaporators are likewise of known design, asdescribed above, for example tubular or plate heat exchangers.

[0106] Heating is effected in a manner which is likewise known,preferably by natural or forced circulation.

[0107] According to the invention, the evaporator surfaces in thepurifying distillation can also be flushed with a purge gas as describedabove.

[0108] The bottom temperature is as a rule 80-150° C., preferably80-140° C., particularly preferably 90-130° C., and the correspondingpressure is 20-500, preferably 30-300, particularly preferably 40-200,mbar. The reflux ratio is from 5:1 to 1:15, preferably from 2:1 to 1:10.

[0109] The low boiler fraction not used as reflux can be recycledcompletely or partly into the transesterification, directly into areactor or via a column attached thereto.

[0110] From 50 to 100%, preferably from 75 to 100%, particularlypreferably from 90 to 100%, of the bottom product can be recycled intothe catalyst removal. The remainder can be passed into the residuetreatment.

[0111] In order to prevent polymer formation in the distillation units,a solution of about 0.5% of phenothiazine and 0.05% of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl in the starting ester isadvantageously sprayed onto the condensers. The stabilization ispreferably effected using the reflux to which 100-1000 ppm ofphenothiazine and 10-500 ppm of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl have been added.

[0112] A particular embodiment of the process comprises subjecting thebottom discharge of the catalyst removal or, if required, a part thereofand, if required, of the purifying distillation to a continuous orbatchwise thermal treatment (cleavage, distillation). This residuetreatment in which desired ester still present is recovered andsimultaneously the Michael adducts are cleaved into the correspondingalcohols and (meth)acrylates and are separated off is not limited.

[0113] The residue treatment can be operated continuously or batchwisein at least one reactor, if required with an attached column, but ispreferably operated continuously.

[0114] In the residue treatment, for example, Michael adducts andpolymers are cleaved.

[0115] These Michael adducts are in general cleavable into theirstarting compounds, for example by thermal and/or catalytic treatment,e.g. in the presence of a suitable catalyst.

[0116] The stream fed to the residue treatment can be thermally treated,with or without the addition of further, for example basic or acidic,catalysts, for cleavage of the Michael adducts present.

[0117] The temperature in the residue treatment is in general from 100to 220° C., preferably from 120 to 200° C., particularly preferably from140 to 180° C., in particular from 150 to 180° C.

[0118] The removal of the low boilers from the residue treatment can besupported by passing through a gas stream which is substantially inertunder the reaction conditions (stripping), e.g. nitrogen as well as anoxygen-containing gas, such as air or air/nitrogen mixtures.

[0119] In this way, in general 50% or more of the useful productspresent (desired ester and starting materials) can be recovered.

[0120] The useful products separated off can then be recycled to thetransesterification reactor, if required via an attached column,preferably to the catalyst removal or purifying distillation.

[0121] Consequently, no special, technically complicated measures arenecessary for minimizing the formation of the Michael adducts during thesynthesis and/or during the working-up.

[0122] The residue of the residue treatment can then be fed again to aresidue treatment or can be disposed of in a suitable manner, forexample by incineration.

[0123] Instead of a residue treatment, the bottom products of theprocess can of course also be disposed of without treatment.

[0124] The basic (meth)acrylate obtained by the process described has apurity of 99.9% or more, according to gas chromatographic analysis.

[0125] The content of saturated secondary components is as a rule <400ppm, and that of N,N′-dimethylpiperazine, ethylene glycoldi(meth)acrylate and vinyloxyethyl (meth)acrylate <100 ppm.

[0126] Advantages of the Process:

[0127] The process can be operated completely continuously and requiresnot more than 3 distillation columns, or four with residue treatment.The capital costs and maintenance/repair costs are therefore relativelylow.

[0128] Industrial n-butyl (meth)acrylate can be used withoutdifficulties in the plant or quality problems occurring.

[0129] The novel process gives a high yield, i.e. small amounts ofresidue and hence little environmental pollution and low productioncosts.

[0130] High purity, i.e. low content of byproducts and hence excellentfurther processing properties.

[0131] The dialkylaminoalkyl (meth)acrylates prepared according to theinvention, in particular dialkylaminoethyl (meth)acrylates andespecially dimethylaminoethyl (meth)acrylates are useful monomers forthe preparation of copolymers. They are used as monomers as such orafter quaternization in the polymerization.

[0132] Conventional quaternizing agents are, for example, benzylhalides, e.g. benzyl chloride, alkyl halides, e.g. methyl chloride,ethyl chloride, methyl bromide, ethylene dichloride or allyl chloride,alkylene oxides, e.g. ethylene oxide, propylene oxide, styrene oxide,isobutylene oxide or vinyloxirane, preferably ethylene oxide orpropylene oxide and particularly preferably ethylene oxide, alkylphosphites or phosphonates, e.g. trimethylphosphite ortriethylphosphite, dialkyl sulfates, e.g. dimethyl sulfate or diethylsulfate, dialkyl carbonates, e.g. dimethyl carbonate, diethylcarbonateor di-n-butyl carbonate, chlorohydrin or epichlorohydrin.

[0133] In particular, those copolymers which contain quaternizedmonomers in the form of polymerized units are used in the treatment ofwater, for example as ion exchange resins or as a component ofmembranes.

[0134] The example which follows illustrates the novel process withoutrestricting it.

[0135] ppm and percentage data used in this document are by weight,unless stated otherwise.

EXAMPLE 1

[0136] 134 g of dimethylaminoethanol, 175 g of n-butyl acrylate (purity99.6%, 0.1% of dibutyl ether, 0.05% of butyl acetate, 0.03% of butylpropionate, 0.04% of water, 0.1% of isobutyl acrylate, 0.01% of acrylicacid and 0.02% of pentyl acrylates), 6 g of titanium tetrabutylate, 71 gof recycled catalyst and 833 g of recycled stream from the purifyingdistillation, which substantially comprised n-butyl acrylate (about 75%)and dimethylaminoethanol (about 17%) were fed per hour in a continuousoperation, to a transesterification apparatus which consisted of atransesterification reactor (effective volume 1.8 l) with externalcirculation evaporator and attached distillation column (20 dual-flowtrays) with condenser and a postreactor (effective volume 1.2 l) withexternal circulation evaporator. The temperature in the reactor was 120°C. and that in the postreactor 123° C. An n-butanol fraction whichmainly comprised butanol (83%) and butyl acrylate (17%) was separatedoff via the top of the column and was condensed. 132 g of the condensatewere discharged and the remainder (about 1400 g) was applied as refluxto the uppermost column tray. 15 g per hour of a solution of 0.5% ofphenothiazine in n-butyl acrylate were sprayed onto the top of thecondenser. In each case 10 l of air/h were blown into the circulationevaporator. The residence time was 2.6 hours.

[0137] The discharge of the postreactor was fed to a distillation unitwhich consisted of a distillation column (8 dual-flow trays) withcirculation evaporator and condenser (bottom temperature 105° C.), andthe high boilers (85 g/h), mainly comprising desired ester (about 65%),tetra(dimethylaminoethyl) titanate (about 30%) and n-butyl acrylate(about 3%), was separated off, about 80% being recycled into theesterification reactor and the remainder (17 g/h) being discharged. Theproduct mixture (about 60% of n-butyl acrylate, about 13% ofdimethylaminoethanol, about 25% of desired ester and about 2.5% ofn-butanol) obtained at the top of the column (69° C./50 mbar) wascondensed and partly (50%) applied as reflux again to the top of thecolumn. For stabilization, a solution (15 g/h) of 0.5% of phenothiazinein n-butyl acrylate was sprayed into the condenser. The remainder of thedistillate (about 1030 g/h) was fed to a further distillation unit whichconsisted of a column having 22 dual-flow trays with side take-off,condenser and circulation evaporator. A mixture of about 16.7% ofdimethylaminoethanol, about 75% of n-butyl 45 acrylate, about 5% ofdesired ester and about 3% of n-butanol was separated off at the top ofthe column and was partly (about 40%) applied as reflux again at the topof the column and partly (833 g/h) recycled into the transesterificationreactor. The stabilization was effected using a solution of 0.5% ofphenothiazine and 0.05% of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl. The bottom product (about10 g/h), mainly desired ester (about 99%), was fed to the catalystremoval. The dimethylaminoethyl acrylate separated off in gaseous formvia the side take-off was condensed (206 g/h) and was stabilized withabout 20 ppm of hydroquinone monomethyl ether.

[0138] In each case 10 l/h of air were passed into the circulationevaporators of the two distillation units.

[0139] The yield of distilled product was 96% (based ondimethylaminoethanol). The purity determined by gas chromatography was99.9%, <20 ppm of dimethylaminoethanol, about 100 ppm of butyl acrylate,about 250 ppm of water, about 300 ppm of dimethylaminoethyl propionateand <10 ppm of dibutyl ether being found as secondary components.

[0140] N,N′-Dimethylpiperazine, ethylene glycol di(meth)acrylate andvinyloxyethyl (meth)acrylate were not detectable.

[0141] The plant was operated for 30 days without polymer problems.

EXAMPLE 2 As Comparison

[0142] The procedure was as in example 1, but without purge gas in thecirculation evaporators and heat exchangers.

[0143] In the course of 5 days, the content of dimethylpiperazine in thedesired ester increased to about 100 ppm and that of vinyloxyethylacrylate to about 20 ppm. After 7 days, the apparatus had to be shutdown owing to fouling of the heat exchanger pipes with polymer.

EXAMPLE 3

[0144] The effect of the residence time on the formation of byproducts(Michael adducts and N,N′-dimethylpiperazine) in the reactor dischargewas investigated.

[0145] The procedure corresponded to the transesterification processdescribed in example 1. The discharge was analyzed by gas chromatographyafter the catalyst had been separated off by hydrolysis and filtration.Residence time Michael adduct N,N′-Dimethylpiperazine 2 hours 2.5% 0.8%3 hours 4.6% 1.5% 5 hours 6.0% 2.5% 7 hours 6.9% 3.1%

EXAMPLE 4

[0146] The procedure was analogous to example 1 but the residence timewas set at 5 hours.

[0147] A yield of 93%, based on dimethylaminoethanol, was obtained andthe content of N,N′-dimethylpiperazine in the desired ester was 60 ppm.

COMPARATIVE EXAMPLE 1

[0148] With continuous operation, 680 parts of dimethylaminoethanol,2016 parts of a mixture of ethyl acrylate and the distillate of ethylacrylate removal, 70 parts of catalyst solution from example 1 and 110parts of recycled catalyst (bottom product of the catalyst removal) werefed per hour to the first reactor of a reactor cascade consisting of tworeactors. The reactors were each equipped with an attached packed columnand a condenser. Heat was supplied via external heat exchangers. Inaddition, 236 parts per hour of the condensate of the low boiler removalwere added via a feed in the middle of the column of the first reactor.The discharge of the second reactor was fed to a container which wasequipped with a circulation evaporator and connected on the gas side tothe column of the second reactor. The reaction temperatures were 110 and115° C., respectively, and the temperature in the container was 119° C.The ethanol formed in the transesterification was discharged as amixture with ethyl acrylate (48% of ethanol) at the top of the reactorcolumns and as condensed. The combined condensates were partly recycledas reflux into the columns (in each case about 2100 parts) and theremainder (806 parts) was discharged and collected for use in the ethylacrylate preparation. The condensates were stabilized by adding 120parts of a solution of 0.5% of phenothiazine and 0.05% of4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl in ethyl acrylate to eachcondenser. The combined condensates contained about 48% by weight ofethanol and about 52% by weight of ethyl acrylate, and thedimethylaminoethanol content was less than 0.1%.

[0149] The discharge of the container was fed to a packed column (fed inthe middle of the column), where it was separated into a top product(top temperature 88° C., 500 mbar) and a catalyst-containing bottomproduct (bottom temperature 140° C.).

[0150] The top product was stabilized by spraying 110 parts of thestabilizer solution described above into the top of the condenser. Thecondensate was mixed with 1170 parts of ethyl acrylate and partly (380parts/hour) recycled as reflux to the column and the remainder (2016parts/hour) was fed to the first reactor.

[0151] The bottom product was fed to a flash evaporator which wasadditionally equipped with a circulation evaporator (135° C., 80 mbar).The distillate was stabilized with 50 parts of stabilizer solution (seeabove) and contained about 90% of dimethylaminoethyl acrylate. Thebottom product was partly (110 parts) fed to the first reactor and theremainder (70 parts/hour) was fed to the residue treatment.

[0152] The ratio of the Michael adduct formation (EP-A 906 902, page 9),calculated over both distillation stages, was negative (−1.3%), whichmeans a partial reduction (cleavage) of the adducts under saidconditions.

[0153] The further working-up of the distillate of the flash evaporationwas carried out analogously to example 2 and gave 1189 parts ofdimethylaminoethyl acrylate with a purity of 99.9%, which corresponds toa yield of 97.9%, based on dimethylaminoethanol.

COMPARATIVE EXAMPLE 2

[0154] The procedure was analogous to comparative example 1 but thebottom temperature in the ethyl acrylate removal was 110° C. and thetemperature in the catalyst removal (flash evaporator) was 110° C.

[0155] The ratio of Michael adduct formation determined on the basis ofthe gas chromatographic analyses of the individual streams was +0.1%,i.e. there was a slight increase in the Michael adduct.

We claim:
 1. A process for the preparation of basic (meth)acrylates IVby transesterification of alkyl (meth)acrylates I in the presence of acatalyst and working-up of the reaction mixture by distillation, whereina gas or gas mixture which is inert under the reaction conditions ispassed through the reaction zone and/or heat exchanger.
 2. A process asclaimed in claim 1, wherein the inert gas or gas mixture is air or anair/nitrogen mixture.
 3. A process as claimed in claim 1 or 2, whereinthe reaction zone comprises a postreactor.
 4. A process as claimed inany of claims 1 to 3, wherein the residence time of the reaction mixturein the reaction zone is from 1.5 to 3 hours.
 5. A process as claimed inany of claims 1 to 4, wherein the transesterification is carried out ina reaction zone having at least one attached column and a nonazeotropicmixture is taken off at the top of the column, in which mixture thecontent of lower (meth)acrylate I is higher compared with the azeotropiccomposition comprising lower alkanol R¹OH and lower (meth)acrylate Iunder the corresponding conditions.
 6. A process as claimed in any ofclaims 1 to 5, wherein the basic (meth)acrylate IV is taken off in thepurification by distillation in gaseous form via a side take-off.
 7. Aprocess as claimed in any of claims 1 to 6, wherein thetransesterification is carried out in the presence of a titaniumtetraalcoholate, the alcoholate corresponding to the alcohol componentR¹OH of the lower alkyl (meth)acrylate I.
 8. A process as claimed in anyof claims 1 to 7, wherein the stabilization of the reaction mixtureduring the working-up is carried out using phenothiazine or a mixture ofphenothiazine and at least one further stabilizer.
 9. A process asclaimed in any of claims 1 to 8, wherein the stabilization of thereaction mixture during the working-up is carried out using a mixture ofphenothiazine and 4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl.
 10. Aprocess as claimed in any of claims 1 to 9, wherein at least a part ofthe distillation bottom products obtained during the working-up issubjected to a catalyst removal.
 11. A process as claimed in any ofclaims 1 to 10, wherein the lower alkyl (meth)acrylate I used for thetransesterification is n-butyl (meth)acrylate.
 12. A process as claimedin claim 11, wherein the n-butyl (meth)acrylate is used in industrialpurity.
 13. A process as claimed in any of claims 1 to 12, wherein thebasic (meth)acrylate IV is dimethylaminoethyl acrylate.
 14. A processfor the preparation of basic dialkylaminoethyl (meth)acrylates bytransesterification of alkyl (meth)acrylates I in the presence of acatalyst and working-up of the reaction mixture by distillation, whereinthe residence time of the reaction mixture in the reaction zone is from1.5 to 3 hours.
 15. The use of dialkylaminoalkyl (meth)acrylatesobtainable by a process as claimed in any of the preceding claims in aquaternization.